1. Technical Field
This invention relates to polyolefin containers and more particularly to polyolefin containers having improved gas barrier properties.
2. Prior Art
In the manufacture of canned foodstuffs, e.g., meat products such as ham, corned beef hash, chili and beef stew, vegetables such as green peas, green beans and corn, the containers, usually metal cans, are filled with the foodstuff, covered with a metal end closure and sealed.
Attempts to use certain inert synthetic resin materials, such as polyethylene and polypropylene for the canning of food stuffs, have encountered the disadvantage that such resin materials are excessively permeable to gases, such as oxygen, and the permeation of oxygen into the container causes an undesirable discoloration and a depreciation in the taste and qualities of the foodstuff.
The high gas permeability characteristics of olefin polymer resins, such as polyethylene and polypropylene, has resulted in containers fabricated from such resins being rejected in the packaging of oxygen sensitive comestibles where, due to the chemical inertness of the resin, these resins might otherwise be employed to great advantage.
The art has devised a number of ways to reduce the gas permeability of polyethylene and polypropylene resins. Included in these methods is the fabrication of the container from a thermoplastic resin wherein an inorganic mineral filler material such as clay or mica, or a gas impermeable resin such as saran is admixed with the olefin polymer resin and a container is molded from the mixture. Examples of this art include U.S. Pat. Nos. 3,463,350, 3,923,190 and 4,102,974.
In U.S. Pat. No. 4,122,147 there is described a preferred method for the manufacture of containers from filled polyolefin resins having improved gas barrier properties which are sealable with metal closures.
In the method disclosed in U.S. Pat. No. 4,122,147, the container is compression molded from a multilayer billet having a plurality of polyolefin layers, a first of these layers being comprised of a polyolefin resin composition exhibiting relatively high flexibility when molded, and a second of these layers being comprised of a polyolefin resin having incorporated therein a filler such as a mineral such as mica or a polymeric material such as saran or polyacrylonitrile, the presence of which reduces the gas permeability of the polyolefin resin. The first and second layers, when heated to a plasticized state and subjected to a compressive force, flow at non-uniform rates, the first layer flowing at a faster rate than the second layer.
In forming a container in accordance with U.S. Pat. No. 4,122,147, the billet is placed in a molding chamber and is compressed between a pair of die members with sufficient force to cause the layers of the billet to flow radially outward from between the die members at a differential rate into a mold cavity defining the sidewalls of the container. As the die members are advanced through the molding chamber, the faster flowing, more flexible, first layer is extruded into the mold cavity ahead of the remaining layers of the billet and forms the flange and exterior surface portions of the container whereas the slower moving second layers form the interior surface portions of the container.
The advancement of the die members through the molding chamber causes a continuous layer of multilayer material to be deposited and solidified within the mold cavity, the walls of which are maintained at a temperature below the solidification temperature of the extruded material. The extruded material, cooled to its solidification temperature, forms an integral solid hollow article having a multilayer side wall and bottom structure which is then ejected from the molding chamber.
Containers fabricated by the process of U.S. Pat. No. 4,122,147 from a polyethylene multilayer billet in which the second layer contains heavy loadings of mica, e.g., 40-50% by weight mica, when sealed with a metal end, have an oxygen leak rate when measured with a Mocon Oxtran 100 instrument of about 0.2 cc/100 in..sup.2 day at 73.degree. F. When saran is used as a filler at 50% loadings, the oxygen leak rate when measured with the Mocon instrument is also about 0.1 cc/100 in..sup.2 day at 73.degree. F.
It has been determined that containers generally require an oxygen leak rate of about 0.05 cc/100 in..sup.2 day at 73.degree. F. or less in order to be considered for the packaging of oxygen sensitive foodstuffs, i.e., foods such as ham which require shelf-lives of about 18-24 months.
Although containers compression molded in accordance with the process of U.S. Pat. No. 4,122,147 have improved gas permeability, the gas permeability requirements for the most oxygen sensitive foodstuffs have still not been effectively met by polyolefin containers of the prior art.